Recombinant clostridia that fix co2 and co and uses thereof

ABSTRACT

The present invention relates a recombinant  Clostridium  expressing one or more heterologous Wood-Ljungdahl (WL) genes. In particular, the recombinant  Clostridium  produces a metabolite at an increased level. The present invention also relates to a method for producing a metabolite by the recombinant  Clostridium.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/381,701, filed Sep. 10, 2010, the content of which is incorporated herein by reference in its entirety for all purposes.

REFERENCE TO U.S. GOVERNMENT SUPPORT

This work is supported by a grant from the U.S. Advanced Research Projects Agency—Energy (ARPA-E) of Department of Energy (DOE) (Award No. 5710002809). The United States has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates generally to recombinant Clostridia expressing one or more heterologous genes required for a functional Wood-Ljungdahl (WL) pathway, and the uses thereof for producing desirable metabolites.

BACKGROUND OF THE INVENTION

Biological carbon dioxide (CO₂) fixation by non-photosynthetic microbes holds a great promise for producing useful products (e.g., chemicals and biofuels) from CO₂ (which requires an electron source as well, such as H₂). Among the non-photosynthetic carbon fixation pathways, the most important one is the Wood-Ljungdahl (WL) pathway in that it is the only linear pathway known to fix carbon. This is practically significant in that CO₂ can serve as the sole carbon source as long as there is an electron source, such as H₂. The WL pathway is employed by anaerobic organisms, most of which are of the Clostridia class, and are broadly known as acetogens that they fix CO₂ and H₂ to produce acetate. These organisms are responsible for the production of about 10% of the 100 billion U.S. tons of acetic acid which is produced annually on earth. Thus, they fix billions of tons of CO₂ on the earth every year and produce 10 billions tons of acetate. Clostridia are Gram-positive, strict anaerobic endospore-formers, are mostly soil organisms, and belong to the phylum Firmicutes. The Clostridia class includes many families, genera (including Clostridium and Moorella), and is made up of hundreds of known species.

The WL pathway is found in over 100 species of anaerobic bacteria, many of which are in the class Clostridia. The best known among them are the acetogenic Clostridia, such as Clostridium aceticum, Clostridium difficile, C. ljungdahli and Moorella thermoacetica (formerly Clostridium thermoaceticum) and, also, Acetobacterium woodii. Besides a strong medical interest in these organisms, Clostridia, with or without a native WL pathway, have been the organisms of choice for the biological production of solvents and butanol. For example, C. acetobutylicum, though without a full and functional WL pathway, is a Gram-positive obligate anaerobe that is well-known industrially for its ability to produce commodity chemicals (e.g., butyrate, acetate, acetoin, and acetone) and biofuels (e.g., butanol and ethanol).

There remains a need to engineer Clostridia and other anaerobes without a functional WL pathway to improve their metabolism of CO₂ as a carbon source for production of useful chemicals and biofuels.

SUMMARY OF THE INVENTION

The present invention relates to recombinant Clostridia expressing one or more heterologous Wood-Ljungdahl (WL) genes. The recombinant Clostridia produce desirable metabolites at an increased level.

According to one aspect of the present invention, a recombinant Clostridium expressing one or more heterologous Wood-Ljungdahl (WL) genes derived from a donor Clostridium is provided. The recombinant Clostridium is derived from a recipient Clostridium. The recipient Clostridium does not have a functional WL pathway while the donor Clostridium has a functional WL pathway. The recombinant Clostridium may have a functional WL pathway. At least one of the one or more heterologous Wood-Ljungdahl pathway genes may be integrated into the genome of the recombinant Clostridium.

The recombinant Clostridium may produce a metabolite at an increased level compared with the recipient Clostridium. The metabolite may be a chemical, biofuel, or biofuel precursor. The chemical may be selected from the group consisting of a carboxylic acid, isopropanol, butanediol, acetoin and propanediol. The carboxylic acid may be butyrate or acetate. The biofuel may be selected from the group consisting of ethanol, n-butanol, i-butanol and 2-butanol.

The recipient Clostridium may be employed to produce a metabolite. The recipient Clostridium may be a solventogenic Clostridium. The solventogenic Clostridium may be selected from the group consisting of C. acetobutylicum, C. beijerinckii, C. saccharoperbutylacetonicum, and C. madisonii. Preferably, the recipient recombinant Clostridium is C. acetobutylicum.

The recipient Clostridium may be a cellulolytic Clostridium. The cellulolytic Clostridium may be selected from the group consisting of C. thermocellum, C. phytofermentans, and C. cellulolyticum. The donor Clostridium may be selected from the group consisting of M. thermoacetica, C. carboxidivorans, C. difficile, and C. ljungdahlii. Preferably, the donor Clostridium is C. difficile or C. ljungdahlii.

According to another aspect of the present invention, a method of producing a metabolite is provided. The method comprises culturing the recombinant Clostridium of the present invention in a culture medium, whereby the recombinant Clostridium produces the metabolite at an increased level compared with the recipient Clostridium. The method further comprise recovering the metabolite from the recombinant Clostridium or the culture medium. The metabolite may be selected from the group consisting of acetate, ethanol, butyrate, acetoin and butanol. The Clostridium is preferably C. acetobutylicum. The donor Clostridium is preferably C. difficile or C. ljungdahlii.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the Wood-Ljungdahl pathway with enzyme annotation adapted from Ragsdale et al. (2008) “The complete genome sequence of Moorella thermoacetica (f. Clostridium thermoaceticum)” Environmental Microbiology 10 (10): 2550-2573). [H] represents a reducing equivalent. Boxed gene IDs indicate the WL pathway genes natively expressed in C. acetobutylicum based on annotation in Kyoto Encyclopedia of Genes and Genomes (KEGG) (Kanehisa and Goto (2000). “KEGG: Kyoto Encyclopedia of Genes and Genomes.” Nucleic Acids Research 28 (1): 27-30).

FIG. 2 shows the WL region in the C. ljungdahlii (A), C. difficile (B), C. carboxidivorans (C), and M. thermoacetica (D) genomes. The region in the Clostridium acetogens is highly conserved in both order and homology. The genes involved in the CO dehydrogenase/acetyl-CoA synthase are also consolidated into one region of M. thermoacetica that has different organization than the other examined clostridia; however, the other M. thermoacetica WL genes are distributed throughout its chromosome. The dashed boxes contain the enzymes important to pathway functionality that are not present in CAC. The solid box indicate genes already present in C. acetobutylicum. The following genes are labeled with their gene product in the: CODH/ACS Subunits (α and β), which comprise the major parts of the CO dehydrogenase/CoA synthase; cooC1 and cooC2, nickel insertion proteins that are required to form the CODH/ACS active sites (Ludden et al. (2001). “Purification and characterization of membrane-associated CooC protein and its functional role in the insertion of nickel into carbon monoxide dehydrogenase from Rhodospirillum rubrum.” Journal of Biological Chemistry 276 (42): 38602-38609); CFeSP Subunits (α and β), which form the corrinoid iron/sulfur protein; MeTr, a methyltransferase that carries the methyl- group from 5-methyltetrahydrofolate to the CFeSP; and Ferredoxin, which may be important in regenerating the metal catalytic sites of the CODH/ACS (Ragsdale and Bender (2011) “Evidence That Ferredoxin Interfaces with an Internal Redox Shuttle in Acetyl-CoA Synthase during Reductive Activation and Catalysis.” Biochemistry 50 (2): 276-286).

FIG. 3 shows promoter activity as determined by the β-galactosidase assay with samples taken at 6 hr, 12 hr, and 24 hr timepoints.

FIG. 4 shows the optimized mazF DNA sequence (SEQ ID NO: 1) useful for a double crossover integration.

FIG. 5 shows the map of plasmid pKRAH1_mazF. catP, chloramphenicol resistance; oriEC, gram negative origin of replication; bgaR, beta-galactosidase regulator; rep, gram positive replication protein; oriCP, gram positive origin or replication; mazF, MazF transcript.

FIG. 6 shows cell survival upon induced expression of mazF. On media lacking lactose, the mazF transcript was not transcribed; therefore cells grew and survived normally. On media containing 40 mM lactose, cells transcribed and translated the optimized mazF gene which was lethal to the cells and hence no colonies were seen on the plate.

FIG. 7 illustrates a chromosomal integration with a non-replicating vector, a chromosomal target of integration, and an integration product.

FIG. 8 shows the map of p94CD0725-27 plasmid. MLS region codes for erythromycin resistance and Ap region codes for amplicillin resistance.

FIG. 9 shows the expression of CD0725, CD0726 and CD0725 in C. acetobutylicum by semi-quantitative RT-PCR on the p94CD0725-27 strain.

FIG. 10 shows cell growth (Abs₆₀₀) profile of the 824(p94CD0725-27) strain and the 824(pSOS94del) plasmid control strains of C. acetobutylicum.

FIG. 11 shows an end point (120 hr) HPLC analysis for supernatant sample of the 824(p94CD0725-27) strain and the 824(pSOS94del) plasmid control strains of C. acetobutylicum.

FIG. 12 shows incorporation of ¹⁴C in C. acetobutylicum (WT and the recombinant strain Cac(p1/3) and the WT C. ljungdahlii.

FIG. 13 shows the map of pJIR750_CD3317728. Cm-R region codes for chloramphenicol/thiamphenicol resistance.

FIG. 14 shows the mRNA expression of CD3317 in C. acetobutylicum by semi-quantitative RT-PCR on the 824(p95CD3317) strain.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that expressing heterogonous genes required for the Wood-Ljungdahl (WL) pathway in a Clostridium not having a functional WL pathway improves metabolism of CO₂ by the Clostridium. The present invention relates to engineering a Clostridium not having a functional WL pathway by expressing heterologous WL genes in the engineered or recombinant Clostridium. The recombinant Clostridium produces desirable metabolites at an increased level.

The term “polynucleotide” used herein refers to a polymer of nucleotide residues with no limitation with respect to the minimum length of the polymer. Preferably, the polynucleotide has at least 60 nucleotides. The polynucleotide may be a DNA, cDNA or RNA molecule.

The term “variant” of a polynucleotide as used herein refers to a polynucleotide having a nucleic acid sequence that is the same as the nucleic acid sequence of the polynucleotide except having at least one nucleic acid modified, for example, deleted, inserted, or replaced, respectively. The variant may have a nucleic acid sequence at least about 80%, 90%, 95%, or 99%, preferably at least about 90%, more preferably at least about 95%, identical to the nucleic acid of the polynucleotide.

The term “derived from” used herein refers to the origin or source, and may include naturally occurring and recombinant microorganisms or molecules, or variants thereof. For example, a gene derived from a bacteria may be identical to the corresponding native gene in the bacteria or a variant thereof, i.e., having a nucleic acid sequence at least about 80%, 90%, 95%, or 99%, preferably at least about 90%, more preferably at least about 95%, identical to the corresponding native gene.

The present invention provides a recombinant Clostridium expressing one or more heterologous Wood-Ljungdahl (WL) genes derived from a donor Clostridium. The recombinant Clostridium is derived from a recipient Clostridium. While the recipient Clostridium does not have a functional WL pathway, the donor Clostridium has a functional WL pathway.

The Wood-Ljungdahl (WL) pathway has two branches: the Eastern or methyl branch, and the Western or carbonyl branch (FIG. 1). Numerous genes encoding various enzymes are required in each branch to constitute a functional WL pathway. A WL pathway is functional in a bacterium if it enables the bacterium to assimilate carbon in CO₂ and/or CO molecules into cellular carbon.

The recombinant Clostridium may produce a metabolite at an increased level compared with the recipient Clostridium. The metabolite may be a chemical, biofuel, or biofuel precursor. The chemical may be selected from the group consisting of a carboxylic acid, isopropanol, butanediol, acetoin and propanediol. The carboxylic acid may be butyrate or acetate. The biofuel is selected from the group consisting of ethanol, n-butanol, i-butanol and 2-butanol. Preferably, the recombinant Clostridium produces acetate, ethanol, butyrate, acetone and/or butanol.

A recipient Clostridium is a Clostridium that does not have a functional WL pathway, and into which one or more heterogonous WL genes are introduced. Preferably, recipient Clostridium is employed to produce a metabolite. For example, it may be a solventogenic Clostridium or a cellulolytic Clostridium. The solventogenic Clostridium may be selected from the group consisting of C. acetobutylicum, C. beijerinckii, C. saccharoperbutylacetonicum, and C. madisonii. The cellulolytic Clostridium may be selected from the group consisting of C. thermocellum, C. phytofermentans, and C. cellulolyticum. More preferably, the recipient recombinant Clostridium is C. acetobutylicum (e.g., ATCC 824).

A donor Clostridium is a Clostridium that has a functional WL pathway, and from which one or more WL genes are obtained. Donor Clostridium may be selected from the group consisting of M. thermoacetica, C. carboxidivorans, C. difficile, and C. ljungdahlii. Preferably, the donor Clostridium is C. difficile or C. ljungdahlii. More preferably, the donor Clostridium is C. ljungdahlii.

A “Wood-Ljungdahl (WL) gene” as used herein refers to a full length gene required for a functional WL pathway in a bacterium, or a functional fragment or variant thereof. Some donor Clostridia (e.g., M. thermoacetica, C. carboxidivorans, C. difficile, and C. ljungdahlii (CLJ)) have been sequenced. A comparative analysis of their genomes may be used to identify the genes required for a functional WL pathway (i.e., WL genes). For example, a comparative genome analysis was performed using the Basic Local Alignment Search Tool (BLAST) to confirm the correct annotations for each of the WL enzymes as well as which gene products form protein complexes with each other. (Altschul et al. (1990). “Basic Local Alignment Search Tool.” Journal of Molecular Biology 215 (3): 403-410). The BLAST comparison among the WL genes of C. ljungdahlii, C. difficile, and C. carboxidivorans has shown highly conserved regions of the genome (FIG. 2) containing the WL genes that catalyze reactions 2 through 8 listed on the diagram in FIG. 1. The amino acid residues across the three organisms were between 62% and 92% identical for the genes in this region. Further comparison of these three genomes to that of M. thermoacetica revealed which gene products form multiprotein complexes. For instance, the genes encoding CODH/ACS subunit α, CODH/ACS subunit β, CFeSP subunit α, and CFeSP subunit β are annotated generically as subunits of the CODH/ACS in the genomes of C. ljungdahlii, C. difficile, and C. carboxidivorans. Given the α- and β-subunits of these proteins, the expression vectors may be designed with both subunits on the same vector and under the same promoter; therefore, variability in promoter strength and plasmid copy number may not alter the relative mRNA levels for each subunit. In addition to the common genes found in the WL region, a formate dehydrogenase (FDH), which catalyzes the first reaction in the Eastern branch, was found in C. ljungdahlii, C. difficile, and C. carboxidivorans.

A recipient Clostridium may lack one or more genes required for a functional WL pathway (i.e., WL genes). A genome comparative analysis of a recipient Clostridium and a donor Clostridium may be used to identify the missing WL genes. For example, C. acetobutylicum does not have a functional WL pathway but does have a number of homologs to components of the WL pathway (Table 1).

TABLE 1 Homologs of the WL pathway in M. thermoacetica, C. difficile and C. acetobutylicum Genes of Wood Ljungdahl pathway in: M. thermoacetica C. acetobutylicum C. difficile Protein identity between: (MTA) (CAC) (CDF) MTA/CAC MTA/CDF CDF/CAC Eastern Moth_2312 — CD3317 — 32.7% — Moth_2314 CAC0764 CD1537 21.5% 26.6% 37.1% Moth_0109 CAC3201 CD0718 64.8% 66.2% 62.1% Moth_1516 CAC2083 CD0720 44.1% 42.1% 37.1% Moth_1191 CAC0291 CD0722 12.5% 38.6% 13.1% Western Moth_1197 CAC0578 CD0727 6.3% 37.6% 6.8% Moth_1201 — CD0726 — 37.8% — Moth_1198 — CD0725 — 38.0% — Moth_1203 CAC2498/0116 CD0716 29.8%/27.5% 38.5% 30.2%/29.8% Moth_1202 — CD0728 — 46.1% —

The first homolog in C. acetobutylicum is the β-subunit of the formate dehydrogenase, which reduces CO₂ to formate in the first reaction in the Eastern branch of the WL pathway. In M. thermoacetica, this enzyme is made up of α and β subunits, Moth_(—)2312 and Moth_(—)2314, respectively. However a potential homolog has been only found for the β-subunit in C. acetobutylicum. For the remaining enzymes in the Eastern branch, good homologs have been found except for one. CAC0291 has poor homology but is a bifunctional enzyme in C. acetobutylicum, which codes for both the needed methylenetetrahydrofolate reductase and a homocysteine S-methyltransferase.

Unlike the Eastern branch, C. acetobutylicum is missing most of the enzymes from the Western branch. The only good homologs which have been found in C. acetobutylicum are two carbon monoxide dehydrogenases. A second potential homolog is a methyltetrahydrofolate methyltransferase, CAC0578, which has very poor protein identity with the corresponding enzyme in both M. thermoacetica and C. difficile. However, CAC0578 is annotated as being able to catalyze the reaction from methyl-H₄folate to H₄folate, the reaction that the M. thermoacetica and C. difficile enzymes carry out. The remaining components of the Wood-Ljungdahl pathway, the corrinoid iron-sulfate protein (CFeSP) and the acetyl-CoA synthase, have no homologs in C. acetobutylicum.

All these homologs identified in C. acetobutylicum (and all genes belonging in the same operon with those) are highly expressed. Expression of these genes may not restrict the ability to institute a functional WL pathway in C. acetobutylicum.

Once the missing WL genes are identified in a recipient Clostridium, some or all of these WL genes may be introduced into and expressed in the recipient Clostridium to generate a recombinant Clostridium using techniques known in the art. Preferably, the recombinant Clostridium contains a full set of WL genes, and has a functional WL pathway. For example, WL genes, including: a formate dehydrogenase (CD3317, 2.1 kb), the CFeSP α-subunit (CD0726, 1.4 kb), the CFeSP β-subunit (CD0725, 0.9 kb), an acetyl-CoA synthase (CD0728, 2.1 kb), a methyltetrahydrofolate methyltransferase (CD0727, 0.8 kb), and the β-subunit of the formate dehydrogenase (CD1537, 1.4 kb), may be introduced into C. acetobutyllcum to generate a functional WL pathway in the C. acetobutyllcum. These WL genes may be derived from C. difficile, which has a functional WL pathway and is a closer relative to C. acetobutylicum than M. thermoacetica. Genomic C. difficile DNA may be purchased from ATCC to amplify all these WL genes using PCR. To enable and/or enhance CO₂ fixation by C. acetobutylicum, additional WL genes such as CD0727 from C. difficile, the β-subunit of the formate dehydrogenase from C. difficile, and/or WL genes from M. thermoacetica genomic DNA may also be introduced into C. acetobutylicum.

In the recombinant Clostridium, a heterologous WL gene may be maintained in a self-replicating plasmid or integrated into the genome of the recombinant Clostridium. When the recombinant Clostridium comprises two or more heterogonous WL genes, some or all of the heterogonous WL genes may be integrated into the genome of the recombinant Clostridium, or carried in two or more co-existing plasmids, which preferably have compatible replication origins.

Two plasmids may be used to express these genes in C. acetobutylicum. In order to stably maintain two plasmids within a cell, the plasmids' replication origins (origins of replication/ORI/replicon) must be compatible to ensure that the cell's replication machinery is not diluted, or become biased, replicating one plasmid or the other. For example, the Gram-positive origin of replication (repL), from the B. subtilis cryptic plasmid pIM13, is compatible with the Clostridium perfringens pIP404 ORI found in the pJIR750 shuttle vector (American Type Culture Collection, Manassas, Va.) in C. acetobutylicum. Wild-type C. acetobutylicum was transformed with plasmid pJIR750 as previously described (Mermelstein and Papoutsakis (1993). “In vivo Methylation in Escherichia-Coli by the Bacillus-Subtilis Phage-Phi-3t-I Methyltransferase to Protect Plasmids from Restriction Upon Transformation of Clostridium-Acetobutylicum Atcc-824.” Applied and Environmental Microbiology 59 (4): 1077-1081). Transformants were plated onto 2×YTG plates supplemented with 5 μg/ml thiamphenicol. Following 36 hours of incubation, the plates were stored at room temperature for 7 days to allow for spore development. Resistant colonies were selected, heat shocked at 80° C. for 10 minutes and grown in 10 ml CGM supplemented with the appropriate antibiotic. The plasmid was then purified and transformed into E. coli TOP10 (Invitrogen) cells. Two hundred microliters of the transformants were plated onto LB plates supplemented with 35 μg/ml chloramphenicol. Resistance colonies were selected, and the plasmids mini-prepped and confirmed by restriction digests. The repL bearing plasmid, pSOS94del, was subsequently transformed into 824(pJIR750). Transformants were plated onto 2×YTG plates supplemented with 5 μg/ml thiamphenicol and 40 μg/ml erythromycin. After 36 hours of incubation, the plates were stored at room temperature for 7 days to allow for spore development. Resistant colonies were selected, heat shocked at 80° C. for 10 minutes and grown in 10 ml CGM supplemented with the appropriate antibiotics. The plasmids were then purified and transformed into E. coli TOP10 (Invitrogen) cells. Two hundred microliters of the transformants were plated onto LB plates supplemented with 35 μg/ml chloramphenicol and another 200 μl were plated onto LB plates supplemented with 50 μg/ml ampicillin. Resistance colonies from each plate were selected, and the plasmids mini-prepped and confirmed by restriction digests. This strategy may be extended to identifying and testing additional plasmids (3 or more) that co-exist if a large number of genes needs to be expressed. The biological principles of co-existing plasmids apply whether the plasmids are 2, 3 or more.

A number of different promoters may be used to overexpress the WL genes in the recombinant strain of C. acetobutylicum. These include the previously described thiolase promoter (p_(thl)) and the phosphotransbutyrylase promoter (p_(ptb)), both derived from C. acetobutylicum, the newly characterized phosphotransacetylase promoter (p_(pta)), derived from upstream region of the phosphotransacetylase gene (pta) in C. acetobutylicum (FIG. 3), and also two promoters (p_(IG1) and p_(IG2)) from C. ljungdahlii that give expression in C. acetobutylicum. As shown in FIG. 3, the p_(thl) and p_(pta) are both highly expressed at all of the sample timepoints. The promoters from CLJ IG1 and IG2 both showed low, but measurable, levels of expression. If needed, additional promoters may be created by isolating the upstream regions of genes with known levels of expression. The promoters that provide the highest expression in C. acetobutylicum are p_(thl), p_(pta), and p_(ptb), which are highly expressed throughout exponential growth, whereas the heterologous promoters derived from the WL region in C. ljungdahlii provide a lower level of expression in C. acetobutylicum. Having promoters with both high and low expression levels allows one to tailor expression and correct imbalanced gene ratios, which has previously been shown as an important tool for improving pathway expression and production.

In order to expand the genome of C. acetobutylicum with genes encoding the WL pathway from C. ljundahlii, a single crossover integration method may be employed using a suicide plasmid. Briefly, a suicide plasmid (i.e., with no origin of replication that is functional in C. acetobutylicum) containing the B. subtilis resolvase gene recU (to enhance recombination efficiency in C. acetubutylicum) nay be constructed. In addition to the recU, the plasmid may contain a sigma factor F (sigF; this is the gene to be disrupted in order to integrate the desirable genes) region of homology, a thiamphenicol resistance marker and a gram negative origin of replication. Clostridial transformation may be performed using techniques know in the art. After transformation, colonies resistant to thimaphenicol may be screened for the integration of the plasmid carrying the heterogeneous WL genes from C. ljundahlii. Instead of sigF several other C. acetobutylicum genes may be used as a site of integration. Such genes may be chosen from a list of non metabolic genes that would not affect the ability of cells to grow well. The sigF only affects the sporulation process and cells with a disrupted sigF gene grow well.

A double crossover integration method may also be adopted by utilizing a novel counter-selection marker MazF under the control of a lactose inducible promoter. The counter-selection marker was developed utilizing a codon optimized mazF gene for optimal expression in C. acetobutylicum. The MazF, a mRNA interferase, has been successfully used as a counter-selection marker in C. acetobutylcum ATCC824. The MazE-MazF complex is a part of the toxin-antitoxin system present in E. coli. The toxin, MazF, is stable while the anti-toxin, MazE, is labile. It was previously reported that MazF was used successfully as a counter-selection marker in B. subtilis. The mazF transcript was synthetically constructed (DNA 2.0) for optimized translation in C. acetobutylicum ATCC 824 (FIG. 4). A lactose-inducible promoter (bgaR) that was adapted from C. perfringens strain 13 was employed in order to exploit the lethality of MazF. Subsequently, it was cloned downstream of the lactose-inducible promoter (bgaR) in plasmid pKRAH1. The newly constructed plasmid containing the lactose-inducible promoter and the codon optimized mazF gene was renamed (pKRAH1_mazF) (FIG. 5). Once expressed and translated, MazF cleaves mRNA at ACA sequences therefore arresting cell growth. The plasmid was electrotransformed in C. acetobutylicum ATCC 824. Once the strain bearing the plasmid was confirmed, colonies were grown in liquid media supplemented with the appropriate antibiotic for 24 hours. Subsequently, equal volumes of cell culture were plated to solid 2×YTG media supplemented with the appropriate antibiotic with and without lactose. After 24 hours of incubation at 37° C. in an anaerobic chamber, the plates were visually inspected for cell growth. Plates containing lactose did not have any colonies while those lacking lactose showed an abundance of cells (FIG. 6). The toxicity of MazF enables screening for cells that have undergone double crossover events while losing the plasmid backbone.

Following a genome and functionality analysis, a complete set of WL genes may be introduced into C. acetobutylicum. The combined size of this set of WL genes required to confer a functional pathway may be ˜15 kb. Three plasmids, each containing a portion of the WL genes, may be constructed: two of them to be integrated into the C. acetobutylicum chromosome and one to be maintained as a replicating plasmid in C. acetobutylicum. The C. acetobutylicum mutant M5 is a degenerated strain of C. acetobutylicum, which has lost the pSOL1 megaplasmid, the ability to sporulate, and the ability to produce butanol and acetone, and may be used for cloning the WL genes because transformation efficiency is higher in M5 than in the C. acetobutylicum type-strain, ATCC 824.

The first chromosomal integration may be performed with a non-replicating vector—that is, a plasmid without a clostridial origin of replication—consisting of a portion of the WL genes under the control of a strong promoter, a homologous sequence targeting the chromosome, an antibiotic selection marker, and a minimal vector backbone. Because the plasmid is unable to propagate in C. acetobutylicum, the only clones that express an antibiotic resistance selection marker would be those in which the vector has integrated. The region of homology on the first integrating vector may target the sigma factor F gene (FIG. 7), a sporulation-related sigma factor that is superfluous in the non-sporulating M5.

The second non-replicating plasmid may also be integrated on the M5 chromosome whereby its region of homology may target and disrupt the selection marker from the first plasmid, allowing the maintenance of two single-crossover integrations with only one antibiotic resistance marker. The vector backbone of the second plasmid may be different from the first, ensuring that the only homologous region is the antibiotic resistance gene. The third plasmid may be maintained as a replicating plasmid. After the cloning steps are completed, the recombinant strain may be cultured under CO₂ and H₂ to determine to what extent the pathway is functional.

A recombinant Clostridium may be maintained or grown in a culture medium. For example, an engineered strain may be grown in serum bottles using a modified Hungate technique (Miller and Wolin (1974). “A serum bottle modification of the Hungate technique for cultivating obligate anaerobes.” Appl Microbiol 27 (5): 985-987) in defined media (Monot et al. (1982). “Acetone and Butanol Production by Clostridium-Acetobutylicum in a Synthetic Medium.” Applied and Environmental Microbiology 44 (6): 1318-1324) under CO₂/H₂. Briefly, small test tubes may be filled with nonsterile, defined media, and gassed using CO₂. After gassing, a butyl rubber stopper may be used to seal the tube and a crimped metal seal may be added. The tube and media may then be autoclaved. Before inoculation, the tubes may be filled with H₂ and CO₂ (80:20, v/v) to a final pressure of 0.2 MPa. Optimal conditions for C. acetobutylicum may be determined empirically using the established art in this field. A defined clostridial medium (Monot et al. (1982). “Acetone and Butanol Production by Clostridium-Acetobutylicum in a Synthetic Medium.” Applied and Environmental Microbiology 44 (6): 1318-1324) is used with minimal glucose (1 to 80 g/L can be used). To inoculate and sample the culture, a hypodermic syringe and needle may be used.

The expression of mRNA from cloned WL genes in C. acetobutylicum may be determined. For example, semi quantitative RT-PCR may be performed on mid-exponential (6 hr), early stationary (12 hr) and stationary (24 hr) phase cultures of the C. acetobutylicum containing heterologously expressed WL genes.

Reverse transcription of total RNA may be carried out with random hexamer primers, using High Capacity cDNA ReverseTranscription Kit (Applied Biosystems). PCR may be performed on 50 ng of cDNA using primers specific to the WL genes. To test for DNA contamination during RNA isolation, equal amount of RNA may be used as a template for PCR using suitable primer sets.

CO-dehydrogenase/Acetyl-CoA synthase protein complex (CLJ_c37670/CLJ_c37550) and corrinoid iron-sulfate protein (CLJ_c37580, CLJ_c37570) form a multi subunit protein complex to be a functional enzyme unit. Therefore, determining mRNA levels of the heterologously expressed genes in C. acetobutylicum and comparing them to mRNA levels in an acetogen that natively expresses WL-pathway genes (C. ljungdahlii), may be instrumental in understanding transcript levels required to incorporate functional WL-pathway enzymes in C. acetobutylicum.

Q-RT PCR may be performed on both C. acetobutylicum strains that express WL-pathway genes and wild type C. ljungdahlii grown on CO₂+H₂. Both C. acetobutylicum and C. ljungdahlii cultures may be harvested at an A₆₀₀˜1.00. RNA isolation followed by Q-RT-PCR may be performed on the samples.

Once the heterogonous WL genes are expressed in C. acetobutylicum, expression of proteins/enzymes from the cloned plasmids may be assessed. To determine functionality of the WL proteins in CAC, enzyme assays are conducted on individual strains containing specific enzymes. For example, formate dehydrogenase (FDH) enzyme assay has been performed on C. acetobutylicum strain expressing FDH enzyme. Specific activity of the formate dehydrogenase in CAC strain expressing FDH was observed to be 0.331 mMmin⁻¹mg⁻¹ (U/mg), in comparison to 0.00 U/mg that was observed in non-recombinant WT-CAC (as it does not have an FDH gene). As a positive control, similar assay has also be performed on wild type C. ljungdahlii grown on CO₂+H₂, which has functionally active WL-pathway enzymes, and FDH activity of 1.474 U/mg was observed. This demonstrates our proof of concept that functional WL-pathway enzymes can be expressed into CAC from other related bacteria.

Cell pellet was re-suspended under anoxic conditions, in appropriate volume of Z buffer for an A₆₀₀˜10-20 in minimum of 500 μl. Components of Z buffer include as detailed previously along with 10 mM dithiothreitol (DTT) and 0.1 mM phenylmethylsulfonyl (PMSF). Cell lysate was prepared under anoxic conditions by vortexing the re-suspended cells in 40 mg of glass beads for 1 min and then incubate on ice for 1 min, for a total of 5 repetitions. Cell debris was removed by centrifugation at 16,000×g for 30 min at 4° C.

Formate dehydrogenase (FDH) activity was determined by performing enzyme activity assay under anoxic conditions. The assay was performed in 50 mM Tris HCl (Ph 8.5) plus 5 mM benzyl viologen as electron acceptor and 20 mM sodium formate as substrate. Reaction was be initiated by adding 10 μl of the cell extract and increase in A₅₇₈ was recorded to determine enzyme activity. Reaction conditions without addition of the substrates may be used as negative controls for the experiment.

Similar enzyme assay experiments may be conducted on other WL proteins like Carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS). The assay may be performed as mentioned above by using 1 atm of 100% CO as a substrate instead of sodium formate.

In order to test for CO₂ uptake and utilization in C. acetobutylicum, two assays may be used. The first assay measures simply if there is growth of the organism in the tubes with only CO₂ and H₂ present, but no other carbon or electron source. In the second assay, the concentration of CO₂ and H₂ in the headspace of the flask throughout the fermentation is measured via gas chromatography. The recombinant strains consume the CO₂ and H₂ while the wild-type control shows minimal to no consumption.

To further enhance CO₂ and H₂ utilization, native C. acetobutylicum genes (Table 1) as well the native hydrogen-uptake genes (namely: CAC0028—hydA, CAC0808-0811—hybG-hypE-hypF-hypD, CAC3230—ferredoxin, CAP0141-0143—mbhS-mbhL-hyaD) may be overexpressed using stronger promoters, like the ptb, thl, and the pta (phosphotransacetylase) promoters. Random chemical mutagenesis and transposon mutagenesis may also be used to screen for a strain that uses CO₂ and H₂ at high rates. After a set of WL genes proves successful in enabling CO₂ and H₂ utilization and at high rates in a recipient Clostridium, these genes may be integrated into the chromosome.

The present invention also provides a method of producing a metabolite. The method comprises culturing the recombinant Clostridium of the present invention in a culture medium under conditions permitting that the recombinant Clostridium produces the metabolite at an increased level compared with the recipient Clostridium. The method further provides recovering the metabolite from the recombinant Clostridium or the culture medium.

In a method according to the present invention, the metabolite may be a chemical, biofuel, or biofuel precursor. The chemical may be selected from the group consisting of a carboxylic acid, isopropanol, butanediol, acetoin and propanediol. The carboxylic acid may be butyrate or acetate. The biofuel is selected from the group consisting of ethanol, n-butanol, i-butanol and 2-butanol. Preferably, the recombinant Clostridium produces acetate, ethanol, butyrate, acetone and/or butanol.

In a method according to the present invention, the recipient Clostridium may be employed to produce a metabolite. For example, it may be a solventogenic Clostridium or a cellulolytic Clostridium. The solventogenic Clostridium may be selected from the group consisting of C. acetobutylicum, C. beijerinckii, C. saccharoperbutylacetonicum, and C. madisonii. The cellulolytic Clostridium may be selected from the group consisting of C. thermocellum, C. phytofermentans, and C. cellulolyticum. More preferably, the recipient recombinant Clostridium is C. acetobutylicum (e.g., ATCC 824).

In a method according to the present invention, the donor Clostridium may be selected from the group consisting of M. thermoacetica, C. carboxidivorans, C. difficile, and C. ljungdahlii. Preferably, the donor Clostridium is C. difficile or C. ljungdahlii. More preferably, the donor Clostridium is C. ljungdahlii.

The term “about” as used herein when referring to a measurable value such as an amount, a percentage, and the like, is meant to encompass variations of ±20% or ±10%, more preferably ±5%, even more preferably ±1%, and still more preferably ±0.1% from the specified value, as such variations are appropriate.

Example 1 Construction of p94CD0725-27

Three genes necessary for the Western branch of the WL pathway: CFeSP α-subunit (CD0726), CFeSP β-subunit (CD0725) and methyltetrahydrofolate methyltransferase (CD0727) were cloned into the pSOS94 vector, under the control of phosphate butyryltransferase (ptb) promoter. Genes CD0725, CD0726 and CD0727 were PCR amplified from C. difficile genomic DNA (ATCC: BAA-1382™) using cd0725_fwd and cd0727_rev primers, excluding the natural promoter. This 3.1 kb PCR product was re-amplified using Fwd_(—)0725_BamHI and Rev_(—)0727_KasI primers, to include BamHI and KasI restriction sites on either ends of the PCR product. The PCR product, digested with BamHI and KasI, was ligated to the linearized pSOS94 vector having suitable sticky ends to generate p94CD0725-27 plasmid (FIG. 8). All primers used for plasmid construction are listed in Table 2.

TABLE 2 Primers used for the construction of p94CD0725-27 and RT-PCR confirmation Primer Name Sequence (5′→3′) CD0725_fwd AGAATTAAAATAGGCTTAAGGGGG (SEQ ID NO: 2) CD0727_rev CTATTTCCCCCTTTAATATTTACACC (SEQ ID NO: 3) Fwd_0725_BamHI AAGGATCCAGGAGGATGGCATTTAAAATGTCTACTCAAAAATA (SEQ ID NO: 4) Rev_0727_KasI AAGGCGCCCTAGAAAGCAAATCCACCTTCA (SEQ ID NO: 5) RT_fwd_cd0725 GCTAATTGTCCAGTAGAGTGGGCT (SEQ ID NO: 6) RT_rev_cd0725 ACTGCCTTAGCTACATCAGCAC (SEQ ID NO: 7) RT_fwd_cd0726 TGCCCACATATGTCTGATGACGCT (SEQ ID NO: 8) RT_rev_cd0726 TACTCAGATGCACCAGCACCAACT (SEQ ID NO: 9) RT_fwd_cd0727 ACTCTGCTGATGCTGGCTCAAGAT (SEQ ID NO: 10) RT_rev_cd0727 TGGCAGTAAGCCATACGCTCATCA (SEQ ID NO: 11)

The p94CD0725-27 plasmid was transformed into TOP10 chemically competent E. coli cells (Invitrogen) according to user guidelines. Sequence and orientation of the insert in the plasmid was confirmed using sequencing reaction. To avoid natural restriction system of C. acetobutylicum, p94CD0725-27 was methylated by transforming it into E. coli ER2275 containing pANI plasmid. Methylated plasmid was then transformed into C. acetobutylicum by electroporation to generate C. acetobutylicum 824(p94CD0725-27) strain (abbreviated as 824(p94CD0725-27)).

Example 2 Expression of mRNA in the 824(p94CD0725-27) Strain

The expression of mRNA from p94CD0725-27 plasmid was determined. Semi-quantitative RT-PCR was performed on mid-exponential (6 hr), early stationary (12 hr) and stationary (24 hr) phase cultures of the 824(p94CD0725-27) strain (FIG. 9). Reverse transcription of total RNA was carried out with random hexamer primers using High Capacity cDNA ReverseTranscription Kit (Applied Biosystems). PCR was performed on 50 ng of cDNA using primers specific to CD0725, CD0726 and CD0727 (Table 2). To test for DNA contamination during RNA isolation, equal amount of RNA was used as a template for PCR using the primer sets as mentioned above. With minimal or no DNA contamination in RNA isolates, the expression of mRNA from p94CD0725-27 plasmid was confirmed (FIG. 9).

Example 3 Cell Growth in the 824(p94CD0725-27) Strain

The 824(p94CD0725-27) strain reached similar cell densities to that of its plasmid control (FIG. 10). Although the 824(p94CD0725-27) strain had similar growth profile in exponential phase and stationary phase, it attained a higher OD (A₆₀₀) than its plasmid control towards the end of 5 day static flask fermentation (FIG. 10). End point HPLC analysis (120 hr) on supernatant samples of the 824(p94CD0725-27) strain showed it produced somewhat higher levels of solvents apparently due to the expression of these transgenes (FIG. 11).

Example 4 Ability of the Cac(p1/3) Strain to Use CO₂ in the Presence of H₂

Test experiments were carried out for ¹⁴C distributions from ¹⁴C-bicarbonate for both C. ljungdahlii (CLJ) and C. acetobutylicum (CAC; WT and strain Cac(p1/3). Cac(p1/3) expressed 3 WL pathway genes, and was the same as the 824(p94CD0725-27) strain described in Examples 1-3.

For primary cultures C. acetobutylicum and C. ljungdahlii were grown on CGM and ATCC 1754, respectively, using fructose as a carbon source. A 10% inoculum from exponentially growing primary culture was transferred to 150 ml anaerobic serum bottles, containing 30 ml of sugar-free media containing 1-2 μCi of sodium [14C]carbonate, for growth on CO₂ as sole source of carbon. Cultures were grown at 37° C. at 110 rpm for 48 hrs before harvesting. CO₂ from the head space, biomass and cell supernatant were collected and analyzed by the scintillating counter ¹⁴C in the biomass represents CO₂ incorporation, ¹⁴C counts in the supernatant include bicarbonate, dissolved CO₂ and metabolites, and gaseous ¹⁴C counts represents the non-assimilated CO₂, which is converted from bicarbonate.

Fractions of ¹⁴C distributed into biomass, gaseous CO₂ and in soluble extracellular products in the medium, plus the errors associated with such measurements are shown in FIG. 12. More CO₂ (¹⁴C labeled) was incorporated and released mainly as end products into the medium, by C. ljungdahlii than by C. acetobutylicum (WT CAC and Cac (p1/3) strain). A small amount of ¹⁴C was incorporated into the biomass in C. acetobutylicum, which is not an artifact of an experimental error since it was also recently shown by others that WT C. acetobutylicum fixes some CO₂. ¹⁴C from bicarbonate was either going into biomass or products produced by the cells (acetate and/or ethanol for C. ljungdahlii or acetate, butyrate, acetone, butanol and ethanol for C. acetobutylicum) or converted to gaseous CO₂. As shown in FIG. 12, relatively little ¹⁴C went into biomass even for C. ljungdahlii, which grew well on CO₂/H₂. Most ¹⁴C went into extracellular products (and thus into the supernatant of the culture). The recombinant C. acetobutylicum strain Cac(p 1/3) incorporated more into extracellular products than the WT CAC, thus demonstrating that the cloned heterologous WL genes are functional in C. acetobutylicum and altering cell metabolism.

Example 5 Construction of pJIR750_CD3317728

Two genes, formate dehydrogenase (CD3317) and acetyl-CoA synthase (CD0728), were cloned into the pJIR750 vector to make a synthetic operon under the expression of thiolase (thl) promoter. The two genes CD3317 and CD0728 were PCR amplified from C. difficile genomic DNA using CD3317_fwd/rev and CD0728_fwd/rev primers, respectively (Table 3).

TABLE 3 Primers used for the construction of pJIR750_ CD3317728 Primer Name Sequence (5′→3′) CD3317_ GGGAATTGTATGGAGAAAAAAATTTTA fwd (SEQ ID NO: 12) CD3317_ GCCTTTCTGCCTTTTATAAATCTTAAATC rev (SEQ ID NO: 13) CD0728_ GGGAAATAGAATGAATCTATATAATATAA fwd (SEQ ID NO: 14) CD0728_ AAATTTTCCATCAATTACATTACAC rev (SEQ ID NO: 15) CD0728_ AAGGCGCCGAATGAATCTATATAATATAATCTTTA KasI_fwd (SEQ ID NO: 16) CD0728_ AAGGCGCCTTACATTACACTTTCCATAGC KasI_rev (SEQ ID NO: 17) CD3317_ AAGGATCCAGGAGGATGGAGAAAAAAATTTTTACAGTTTG BamHI_fwd (SEQ ID NO: 18) CD3317_ AAGGCGCCTTAGATATTTGTAGCTGTCATTTT KasI_rev (SEQ ID NO: 19)

CD3317 and CD0728 were re-amplified to include BamHI and KasI restriction sites to 5′ and 3′ ends of the PCR product, respectively (Table 3). The CD3317 gene, digested with BamHI and KasI was ligated into linearized pSOS95del vector under thl promoter, to generate p95CD3317 plasmid. The CD0728 gene, digested with KasI, was ligated to linearized p95CD3317 vector with compatible sticky ends to generate p95CD3317-728. The synthetic operon along with thl promoter was fragmented out of the p95CD3317-728 by digesting the plasmid with SalI. This vector fragment was ligated into linearized destination vector, pJIR750, with compatible sticky, to generate pJIR750_CD3317728 (FIG. 13).

p95CD3317 plasmid that is described above was methylated and transformed into C. acetobutylicum by the methods mentioned above for p94CD0725-27, to generate C. acetobutylicum 824(p95CD3317) strain.

Expression of mRNA from p95CD3317 plasmid was determined by performing a semi-quantitative RT-PCR on mid-exponential (6 hr) and early stationary (12 hr) phase cultures of the 824(p95CD3317) strain (FIG. 14) using methodology described in Example 2. To test for DNA contamination during RNA isolation, equal amount of RNA was used as template for PCR as described above for p94CD0725-27. In a cell growth test as described in Example 3, the 824(p95CD3317) strain reached similar optical densities to that of 824(p94CD0725-27) strain (FIG. 10). 

What is claimed:
 1. A recombinant Clostridium expressing one or more heterologous Wood-Ljungdahl (WL) genes derived from a donor Clostridium, wherein the recombinant Clostridium is derived from a recipient Clostridium, wherein the recipient Clostridium does not have a functional WL pathway, and wherein the donor Clostridium has a functional WL pathway.
 2. The recombinant Clostridium of claim 1, wherein the recombinant Clostridium has a functional WL pathway.
 3. The recombinant Clostridium of claim 1, wherein the recombinant Clostridium produces a metabolite at an increased level compared with the recipient Clostridium.
 4. The recombinant Clostridium of claim 3, wherein the metabolite is a chemical, biofuel, or biofuel precursor.
 5. The recombinant Clostridium of claim 4, wherein the chemical is selected from the group consisting of a carboxylic acid, isopropanol, butanediol, acetoin and propanediol.
 6. The recombinant Clostridium of claim 5, wherein the carboxylic acid is butyrate or acetate.
 7. The recombinant Clostridium of claim 4, wherein the biofuel is selected from the group consisting of ethanol, n-butanol, i-butanol and 2-butanol.
 8. The recombinant Clostridium of claim 1, wherein the recipient Clostridium is employed to produce a metabolite.
 9. The recombinant Clostridium of claim 1, wherein the recipient Clostridium is a solventogenic Clostridium.
 10. The recombinant Clostridium of claim 9, wherein the solventogenic Clostridium is selected from the group consisting of C. acetobutylicum, C. beijerinckii, C. saccharoperbutylacetonicum, and C. madisonii.
 11. The recombinant Clostridium of claim 1, wherein the recipient Clostridium is C. acetobutylicum.
 12. The recombinant Clostridium of claim 1, wherein the recipient Clostridium is a cellulolytic Clostridium.
 13. The recombinant Clostridium of claim 12, wherein the cellulolytic Clostridium is selected from the group consisting of C. thermocellum, C. phytofermentans, and C. cellulolyticum.
 14. The recombinant Clostridium of claim 1, wherein the donor Clostridium is selected from the group consisting of M. thermoacetica, C. carboxidivorans, C. difficile, and C. ljungdahlii.
 15. The recombinant Clostridium of claim 1, wherein the donor Clostridium is C. difficile or C. ljungdahlii.
 16. The recombinant Clostridium of claim 1, wherein at least one of the one or more heterologous Wood-Ljungdahl pathway genes is integrated into the genome of the recombinant Clostridium.
 17. A method of producing a metabolite, comprising (a) culturing the recombinant Clostridium of claim 1 in a culture medium, whereby the recombinant Clostridium produces the metabolite at an increased level compared with the recipient Clostridium, and (b) recovering the metabolite from the recombinant Clostridium or the culture medium.
 18. The method of claim 17, wherein the metabolite is selected from the group consisting of acetate, ethanol, butyrate, acetoin and butanol.
 19. The method of claim 17, wherein the recipient Clostridium is C. acetobutylicum.
 20. The method of claim 17, wherein the donor Clostridium is C. difficile or C. ljungdahlii. 